The requirement that asks to be a registered company/business is for the ReWire license, not the VST license. There's been other open-source software with VST support I think, so there must be a way around. For the ReWire, it's gonna be something more tricky. I guess I can handle that part on my own then - and pass my own interface to the developers. Considering the flexible nature of MT3, that shouldn't be too much of a problem.

Having recently noticed the problems arising from licensing decisions in some sound related software, I got interested what's the status of the license for MT3? I noticed in the sourceforge pages that the license info is "GNU General Public License (GPL), GNU Library or Lesser General Public License (LGPL)". The thing is that VST related files from Steinberg can't be used in GPL program, so in practice this means that GPL programs can't provide VST support. Is MT3 really going to be GPL and drop VST support?

LGPL allows linking with closed-source software as far as I understood it. Since MT3 is modular, having it LGPL will allow closed-source plugins to be developed for it.

LGPL libraries can indeed be linked with closed source applications, but using LGPL for application still seems bit odd; in sections 2. a) of the LGPL 2.1 license it reads: "The modified work must itself be a software library.".